This application relates to gas sensing. More particularly, it is concerned with gas sensors employing solid electrolyte material which ionically conducts in the presence of a gas.
Various techniques and apparatus have been developed for determining the concentration of individual gases, such as oxygen, in a gas mixture, such as an exhaust gas. With some of these techniques it is difficult to determine the oxygen concentration in the range from about 0.1% to 20%. With certain types of gas sensors it is difficult to maintain accuracy over a period of time. Some techniques provide output information which varies logarithmically rather than linearly with the concentration of oxygen in the gas mixture. Some sensors are insensitive to slight changes in the partial pressure of oxygen, and therefore subject to inaccuracy. Other techniques involve complex electronic circuitry for controlling the operation of the sensing apparatus and for carrying out the measurements.
In one technique a solid electrolyte material which exhibits ionic conduction in the presence of oxygen is employed in an electrochemical gas pump. The oxygen concentration in the gas mixture is determined by the diffusion-limited current flow through the solid electrolyte material. This technique provides a signal output which is linearly proportional to the oxygen concentration in the gas mixture. A simple power supply provides a constant voltage to the electrodes of the pump. A series resistor is used to generate an output signal which is proportional to the concentration of oxygen in the gas being analyzed.
Sensors of this type require energy in the form of an applied voltage. The applied voltage needed is a function of current density, temperature and oxygen concentration. If the applied voltage is low or the temperature is low or the oxygen concentration is high, the relationship observed between the pumping current and the oxygen concentration is non-linear. If the applied voltage is too high or the temperature is too high or the oxygen concentration is low, other oxygen-containing ingredients such as H.sub.2 O and CO.sub.2 or the solid electrolyte material itself may dissociate contributing to faulty current in the output signal. In addition the pumping of the oxygen through the solid electrolyte material consumes energy, and the electrolyte ohmic polarization of the material is also a function of temperature and the current density. Thus, at high current density or low temperature extra voltage is needed to overcome the resistance of the electrolyte material.
Because of these problems, previously available sensors based on this technique have a limited temperature range of operation. In addition the electrodes of some sensors are exposed to gas flow. After a period of use the electrodes can shift the amount of applied voltage required, and thus lead to errors in the output signal. Heating elements are required in order to maintain a proper operating temperature for the sensor. Heating elements tend to have short lifetimes due to the mechanical instability of their materials at the high operating temperature. Certain devices have pressure-dependent output signals because of the diffusion mechanism involved in the operation of the gas pump. Most of the devices presently available require fairly high voltage in order to be operable, thus creating the possibility of faulty dissocation currents at the resulting high temperature.